Exposure apparatuses are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes a frame assembly, a measurement system, a control system, an illumination source, a projection optical assembly, a reticle stage for retaining a reticle, and a wafer stage for retaining a semiconductor wafer.
The frame assembly typically supports the measurement system, the illumination source, the reticle stage, the projection optical assembly, and the wafer stage above the ground. The measurement system monitors the positions of the stages relative to a reference such as the projection optical assembly. The projection optical assembly projects and/or focuses the light that passes through the reticle. One or more movers precisely position the reticle stage relative to the projection optical assembly. Similarly, one or more movers precisely position the wafer stage relative to the projection optical assembly.
The size of the images and the features within the images transferred onto the wafer from the reticle are extremely small. Accordingly, the precise positioning of the wafer and the reticle relative to the optical assembly is critical to the manufacture of high density, semiconductor wafers.
Unfortunately, mechanical vibrations and deformations in the frame assembly of the exposure apparatus can influence the accuracy of the exposure apparatus. For example, the movers utilized to move the stages generate reaction forces that vibrate and deform the frame assembly of the exposure apparatus. Further, the ground can transfer vibration to the frame assembly.
The vibrations and deformations in the frame assembly can move the stages and the projection optical assembly out of precise relative alignment. Further, the vibrations and deformations in the frame assembly can cause the measurement system to improperly measure the positions of the stages relative to the projection optical assembly. Additionally, vibration of the projection optical assembly can cause deformations of the optical elements within the projection optical assembly and degrade the optical imaging quality. As a result thereof, the accuracy of the exposure apparatus and the quality of the integrated circuits formed on the wafer can be compromised.
One attempt to solve this problem involves the use of one or more air mounts to secure the frame assembly to the ground. The air mounts utilize a cushion of pressurized air to reduce the effect of vibration of the ground causing vibration to the frame assembly. Similarly, one or more air mounts can be used to support the components of the exposure apparatus on the frame assembly. Unfortunately, existing air mounts with adequate damping capacity have a relatively high natural frequency and are relatively stiff.
In light of the above, there is a need for an exposure apparatus with an improved isolation system. Additionally, there is a need for a vibration isolator with sufficient capacity that has a relatively low natural frequency and is not as stiff as air mounts with comparable capacity. Further, there is a need for an exposure apparatus capable of manufacturing precision devices, such as high density, semiconductor wafers.